Fluid catalytic cracking

ABSTRACT

A fluid catalytic cracking apparatus wherein the reaction zone comprises one or more risers and a reaction vessel. A constant inventory of catalyst is maintained in a fluidized state in the reaction vessel and catalyst is continuously withdrawn from the reaction at a rate equal to the rate catalyst enters the reaction vessel. An improved catalyst check valve connecting the reaction vessel and stripping zone controls the catalyst inventory and catalyst withdrawal rate.

United States Patent [191 Bunn et al.

[ l Jan. 8, 1974 FLUID CATALYTIC CRACKING [75] Inventors: Dorrance P.Bunn; Henry E. Jones,

both of Houston, Tex.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Sept. 20, 1971 [21] Appl. No.: 182,030

[52] US. Cl. 23/288 S, 208/164, 137/512, 137/527.8 [51] Int. Cl B0lj9/20 [58] Field of Search 23/288 S; 208/164, 208/163 [56] ReferencesCited UNITED STATES PATENTS 2,587,554 2/1952 Weikart 23/288 S 2,900,3298/1959 Osborne et al. 23/288 S X Slyngstad et al... 23/288 X Wilson,Jr..'.. 23/288 S VIRGIN OIL ""7 3,394,076 7/1968 Bunn, Jr. et al. 23/288S X 3,661,799 5/1972 Cartmell l. 23/288 S X 3,687,841 8/1972 Saxton etal.... 23/288 S X 3,690,842 9/1972 Lockwood 23/288 S X PrimaryExaminer-Morris O. Wolk Assistant ExaminerR. E. Serwin Att0rney-Thomasl-l. Whaley et al.

[5 7] ABSTRACT A fluid catalytic cracking apparatus wherein the reactionzone comprises one or more risers and a reaction vessel. A constantinventory of catalyst is maintained in a fluidized state in the reactionvessel and catalyst is continuously withdrawn from the reaction at arate equal to the rate catalyst enters the reaction vessel. An improvedcatalyst check valve connecting the reaction vessel and stripping zonecontrols the catalyst inventory and catalyst withdrawal rate.

3 Claims, 2 Drawing Figures PRODUCT to F RA C TIONAT/UN 1 FLUIDCATALYTIC CRACKING BACKGROUND OF THE INVENTION This invention relates toa method and apparatus for fluid catalytic cracking of hydrocarbon oils.More particularly, this invention relates to a method for maintaining acatalyst inventory within a reaction zone and to an apparatus formaintaining a constant catalyst inventory in the reaction zone.

In a fluid catalytic cracking process, hydrocarbon oils are reacted inthe presence of a catalyst under condi tions such that a portion of thehydrocarbon oils are converted to desired product. During thehydrocarbon conversion, coke is deposited upon the catalyst. Catalyst isremoved from the reaction zone, from which it may be transferred to astripping zone. In the stripping zone, occluded hydrocarbons are removedfrom the catalyst employing a stripping vapor such as for example steam.The stripping vapors and volatilized hydrocarbons are transferred fromthe stripping zone into the reaction zone from which they aresubsequently recovered as components of the reaction zone hydrocarbonproduct. Stripped catalyst from the stripping zone is transferred to aregeneration zone wherein at least a.

portion of the coke is removed by contacting said stripped catalyst withan oxygen containing gas which causes combustion of the coke andregeneration of the catalyst. Regenerated catalyst is mixed withadditional hydrocarbon oil to be converted in the reaction zone.

According to one method for converting hydrocarbons, regeneratedcatalyst and hydrocarbon vapors are combined near the bottom of a riserunder reaction conditions. The catalyst-hydrocarbon mixture flows upwardin the riser and is subsequently discharged into a reaction vessel. Inthe reaction vessel, hydrocarbon vapors are separated from the catalyst.In this method of cracking hydrocarbon oils, one or more risers may beemployed. For instance, in U. S. Pat. Nos. 3,394,076 I and 3,433,733,fluid catalytic cracking methods are described wherein two risers areemployed. In the first riser, fresh hydrocarbon oil feed is combinedwith regenerated catalyst for reaction therein. In the second riser,recycle oil, comprising relatively high boiling components obtained fromthe cracked hydrocarbon product of the catalytic cracking reaction, iscombined with regenerated catalyst and subjected to an additionalcracking reaction.

In the reaction vessel, a bed of catalyst is maintained in a fluidizedstate by the passage of vapors therethrough. The fluidized catalyst bedsegregates itself into a lower dense phase wherein the density of thecatalyst bed is from about 0.5 to about 0.9 times the bulk density ofthe unfluidized catalyst, and into a dilute phase which has a solidsconcentration of only about 0.1 to 0.3 lbs. per cubic foot. Hydrocarbonvapors en tering the reaction zone via the riser or risers provide asubstantial proportion of the vapors required to maintain the catalystbed in a fluidized state. Additional vapors may comprise primarystripping steam near the bottom of the dense phase which may be addedfor the purposes of separating a portion of the occluded hydrocarbonsfrom the catalyst and maintaining the dense phase in a fluidized state.

Catalyst is continuously discharged from the risers into the reactionvessel, and catalyst is continuously withdrawn from the dense phase inorder to maintain a desired catalyst inventory within the reactionvessel.

The rate at which catalyst enters the reaction zone is a function of twoseparately controlled reaction variables; the ratio of catalyst tohydrocarbon in the reaction mixture and the flow rate of hydrocarbon oilin the risers. Therefore. the rate at which catalyst enters the reactionvessel depends upon the selected ratio of catalyst to hydrocarbon andalso upon the flow rate of hydrocarbon in the risers. In order tomaintain a selected inventory of fluidized catalyst in the reactionvessel, it is necessary to withdraw catalyst from the dense phase atsubstantially the same rate at which catalyst enters the reaction vesselvia the risers. According to methods of the prior art, the inventory ofcatalyst within the reaction vessel is maintained at a selected value bymeasuring a pressure differential between a point near the bottom of thedense phase and a point above the dense phase and controlling the rateat which catalyst is withdrawn from the dense phase to maintain themeasured pressure differential at a preselected value. This method forcontrolling the catalyst inventory is accomplished by installing apressure tap in the reaction vessel near the bottom of the dense phaseand installing a pressure tap in the reaction vessel in the dilutephase. These pressure taps are connected to a differential pressuretransmitter with piping or tubing. The differential pressure transmitterprovides an output signal proportional to the difference in pressurebetween the lower tap and the upper tap. The output signal from thedifferential pressure transmitter is supplied to a control instrumentwhich compares the measured differential pressure with a preselected setpoint value. Should the measured differential pressure vary from the setpoint. the control instrument supplies a signal to a valve actuatorwhich is connected to a slide valve installed inthc dense phase catalystdrawoff means. The valve actuator responds to the signal from thecontrol instrument by adjusting the position of the slide valve. Thus,the rate at which dense phase catalyst is removed from the reactionvessel is varied in order to maintain the desired differential pressureacross the fluidized catalyst bed.

In a fluidized catalyticcracking process such as described hereinabove amajor portion of the cracking reaction may occur in the risers and aminor portion of the cracking reaction occurs in the reaction vessel.Control of the cracking reaction may be maintained to give a desiredrange of cracked hydrocarbon products by varying reaction varibles suchas hydrocarbon space velocity in the riser, reaction temperature, andthe catalyst to oil ratio. In such a process it has been foundconvenient to maintain the catalyst inventory in the reaction vessel ata constant value.

Catalyst removed from the dense phase in the reaction vessel has cokedeposited upon it which adversely affects its catalytic activity. Also,even where primary stripping is provided in thereaction vessel, suchdense phase catalyst also has appreciable amount of relatively highboiling hydrocarbon liquid occluded thereon. The catalyst removed fromthe reaction vessel is passed into a regeneration zone wherein coke andany other combustible materials are removed by combustion with an oxygencontaining gas such as air. The removal of coke and other combustiblesfrom the catalyst restores its catalytic activity thereby making itsuitable for further use in the fluidizied catalytic cracking process.Rather than burning, and thereby losing, the heavy oils occluded withinthe catalyst withdrawn from the reaction vessel, it is preferable torecover a substantial proportion of such occluded oil by stripping thewithdrawn catalyst with a stripping vapor prior to passing such catalystto the regeneration zone. Accordingly, catalyst withdrawn from thereaction vessel may be passed through a secondary stripping zone whereinit is intimately contacted with a stripping vapor, preferably steam. Amajor portion of the occluded liquid hydrocarbons are thereby vaporizedand stripped from the catalyst. The secondary stripping vapor andvaporized hydrocarbons may conveniently be passed into the reactionvessel at a point above thefluidized catalyst bed via a stripper ventline. By employing this means for disposing of the stripping vapors andvaporized hydrocarbon, the vaporized hydrocarbon may be convenientlyrecovered along with the hydrocarbon product from the reaction vessel.Additionally, by passing such vapors from the secondary stripping zoneinto the reaction vessel above the fluidized catalyst bed, the pressuredifferential between the reaction vessel and the secondary strippingzone is limited to the pressure drop through the stripping vent line.This pressure drop is small and may be controlled by properly sizing thecross section area of the stripping vent line. Therefore, since thepressure differential between the reaction zone and the secondarystripping zone is small, the removal of catalyst from the reactionvessel into the secondary stripping zone is not complicated by largedifferences in pressure between the two zones.

SUMMARY OF THE INVENTION Now, according to the method of the presentinvention, a fluidized catalytic cracking process is provided wherein apreselected inventory of catalyst is maintained in the reaction vessel,wherein catalyst is removed from the reaction vessel at a rate equal tothe rate at which catalyst is added to said reaction vessel and whereincatalyst removed from the reaction zone is regenerated and recycled forfurther use in the process. More particularly, an improved apparatus isprovided for maintaining the inventory of fluidized catalyst within thereaction zone and controlling the rate at which such catalyst iswithdrawn from the reaction zone at a rate equivalent to that at whichcatalyst enters said reaction zone.

By employing the present invention, the two pressure taps, thedifferential pressure instrument, the control instrument, the valveactuator, and the slide valve which were employed in prior art catalystlevel control devices may be eliminated. The elimination of suchequipment results in a substantial cost saving and maintenance of suchequipment is eliminated. Additionally, the rate of withdrawal ofcatalyst from the reaction zone is responsive to the rate of addition ofcatalyst to said zone without reliance upon intravening instrumentationwhich is subject to error and perhaps failure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 of the drawings is a schematicdiagram of a fluidized catalytic cracking process employing theimprovement of the present invention.

FIG. 2 of the drawings is a schematic drawing in cross section showingin more detail a portion of the system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION:

It is known that recycle stocks such as cycle gas-oil separated from thefluid catalytic cracking products are more refractory than virgin chargestock. Preferably, these refractory recycle stocks are cracked undermore severe conditions than virgin stocks. It is also known that a shortperiod of good contact between a charge stock and the catalyst resultsin superior yields as compared to a longer period of poor contact.Accordingly, fluid catalytic cracking processes have been devisedwherein virgin charge stock to the process may be reacted under one setof conditions and recycle charge stock to the process reacted under moresevere conditions. Various apparatus configurations have been proposedfor obtaining different cracking conditions for virgin charge stock andfor recycle charge stock and to achieve more intimate catalyst-oilcontact. In one configuration catalyst and virgin charge stock arecharged to a first riser and effluent from said first riser isdischarged into a reactor vessel at an intermediate point above thecatalyst dense phase. Catalyst and recycle hydrocarbon are charged intoa second riser and the hydrocarbon-catalyst mixture is discharged fromthe second riser into the reactor vessel below the surface of thecatalyst dense phase. Thus, the recycle vapor, by passing through thedense phase catalyst, has an extended contact with the catalyst. Byemploying such an apparatus configuration the virgin charge stock may besubjected to cracking conditions for a relatively short period of goodcontact with the cracking catalyst. The more refractory recycle chargeis maintained in the presence of the cracking catalyst for a longertime.

When operating a fluid catalytic cracking process employing such anapparatus configuration, it has been found that the catalyst inventoryin the reaction zone may not be substantially altered without changingthe depth of the dense phase bed. Substantial changes in the depth ofthe dense phase bed may lead to undesirable consequences. For example,if the dense phase bed inventory increases the virgin charge stock riseroutlet may become submerged in the dense phase, thereby increasing thecontact time of the virgin charge stock with catalyst and resulting indecreased yields of desirable cracked products. On the other hand, whenthe dense phase bed inventory is decreased, the recycle charge stockdischarging into the dense phase may not have sufficient contact timewith the catalyst to obtain proper cracking. Therefore, it has beenfound convenient to operate such a fluid catalytic cracking process witha relatively constant catalyst inventory in the reaction zone.

According to the method of the present invention an improved catalyticcracking process is disclosed wherein the catalyst inventory within areactor vessel is maintained constant employing improved means fortransferring catalyst from the reactor vessel to a secondary strippingzone. The method of the present invention may be better understood byreference to FIG. I of the drawing which figure illustrates oneembodiment by which the method of the present invention may bepracticed. It is not intended to restrict the invention by said FIG. 1,since modifications may be made within the scope of the claims withoutdeparting from the spirit thereof.

Referring to FIG. 1 of the drawing, a virgin gas oil in line 10 iscontacted with hot regenerated catalyst from standpipe 11 at atemperature of about 1,200 F. in the inlet portion of a fresh feed riser12. The resulting suspension of catalyst in oil vapor at a temperatureof about 920 F. and at an average velocity of about 33 ft.

per second passes upward through fresh feed riser 12 and into reactorvessel 15. Fresh feed riser 12 terminates in a downwardly directedoutlet. Conditions prevailing in the fresh feed riser include a catalystto oil weight ratio of 5.6 and a weight hourly space velocity of 69.5.The vapor velocity in the fresh feed riser 12 is about 40 ft. per secondproviding a residence time of about 4.0 seconds. Substantial conversionof the fresh feed occurs in the riser and at these conditions amounts toa conversion of 32 weight percent of the fresh feed into product boilingbelow 430 F.

A heavy cycle gas oil fraction, separated from the cracked product infractionation equipment not shown, having a gravity of about 22 API andan end point tem perature of about 725 F. is introduced through line 20into the inlet section of a recycle riser 21 wherein it is contactedwith hot catalyst from standpipe 22. The resulting catalyst oil vapormixture at a temperature of about 920 F. passes upward through therecycle riser 21 at an average velocity of about 28 feet per second withan average residence time of about 5.0 seconds. Other conditions in therecycle riser include a catalyst to oil weight ratio of 6.2 and a weighthourly space velocity of 51.8. About 16 percent of the gas oil recycleis converted to products boiling below 430 F. by the time the productsare discharged through the outlet of recycle riser 21 into the lowerportion of the reactor vessel 15. The vapor effluent of the recycleriser 21 passes upwardly through a dense phase catalyst bed in reactoraffecting further conversion of the recycle gas oil into 39 percentproducts boiling below 430 F. Other conditions in the dense phase bed inreactor 15 include a catalyst to oil ratio of 12.3 and a weight hourlyspace velocity of 3.0. The combined fresh feed riser cracking, recycleriser cracking and reactor bed cracking provide an overall conversion of70 volume percent of the fresh feed into products boiling below 430 F.The vapor velocities in the reactor vessel 15 are 1.7 feet per second atthe point at which the recycle riser 21 discharges, 3.1 feet per secondat the point where the fresh feed riser l2 discharges and 1.5 feet persecond in the upper portion of the reactor vessel 15.

Cracked product vapors disengage from the catalyst dense phase bed atlevel 18. The level of the dense phase bed 18 is maintained below thedischarge of the fresh feed riser 12 thereby allowing cracked fresh feedvapors to disengagethe catalyst without an extended catalyst contacttime which would result from passing such vapors through the dense phasebed. The desired level 18 of the dense phase bed is obtained bymaintaining a constant catalyst inventory within the reactor vessel 15and controlling the catalyst fluidization vapor velocity through saiddense phase bed, as will hereinafter be further described.

The vapors and entrained catalyst passing upward through the reactorvessel 15 enter cyclone 26 wherein entrained catalyst is separated fromthe vapors and re turned to the catalyst bed through dip leg 29.Although a single cyclone is shown for clarity, it will be understoodthat several cyclones may be assembled in series to achievesubstantially complete separation and a plurality of such assemblies maybe employed to handle the volume of vapor encountered. Effulent vaporspass from cyclone 26 through line 28 into a plenum chamber 30. From theplenum chamber 30, vapors are discharged from the reactor vessel 15through line 31.

Vapor line 31 conveys the hydrocarbon vapors to a fractionaldistillation zone, not shown, wherein the vapors are separated intodesired product and recycle streams by methods well known in the art.

Steam in line 35 is passed to steam ring 37 and discharges near thebottom of the reactor vessel 15 at a point just below the inlet ofcatalyst withdrawal standpipe 38. The steam discharged from steam ring37 and the recycle vapors discharged from recycle riser 21 providevapors to maintain the dense phase catalyst bed in a fluidized state. Ata selected recycle gas oil vapor rate, the level 18 of the dense phasecatalyst bed may be adjusted while maintaining a constant catalystinventory in the reactor vessel 15 by adjusting the rate of steamdischarge through steam ring 37.

Dense phase catalyst in the lower portion of reactor 15 passesdownwardly through the standpipe 38 and a counterweighted check valve 39into a stripping zone 42. The catalyst valve 39 is equipped with counterweights arranged to exert a closing force upon the valve. The weight ofcatalyst collected in the standpipe 38 and in the dense phase bed exertan opening force upon catalyst valve 39. In the operation of a fluidizdcatalyst cracking process, the inventory of catalyst as discharged fromthe risers 12 and 21 will increase in the reactor vessel 15 until thehead of catalyst above the catalyst valve 39 overcomes the closing forceexerted by the counterweights. When the head of catalyst balances theforce of thecounterweights, additional catalyst added to the reactorvessel 15 will cause the catalyst valve 39 to open and allow catalyst topass from the reaction vessel 15 to the stripping zone 42 therebyrestoring the balance between the head of catalyst above the catalystvalve 39 and the closing force of the counterweights. [n the operationof a fluidized catalytic cracking process wherein catalyst iscontinuously entering the reaction vessel 15, the catalyst valve 39continuously passes catalyst from the reaction vessel 15 to thestripping zone 42 to maintain a constant catalyst inventory in thereactor vessel 15. i

In the stripping zone 42 baffles are attached to the wall of saidstripping zone 42. Steam in line 50 is discharged through steam ring 51into the lower portion of the stripping Zone 42 below the baffles 43.Steam rising through the stripping zone 42 vaporizes and separatesoccluded and entrained hydrocarbons from the catalyst entering thestripping zone 42 via the catalyst valve 39. Steam and vaporizedhydrocarbons pass upwardly from the stripping zone 42 through a strippervent line 53 discharging into the upper portion of the reactor vessel 15above the fluidized catalyst dense phase level.

Stripped catalyst is withdrawn from the bottom of the stripping zone 42through a spent catalyst standpipe 55 at a rate controlled by slidevalve 56 and discharges through standpipe 57 into regenerator 58. Inregenerator 58 the spent catalyst is contacted with air introducedthrough line 60 and air ring 61 whereupon coke is burned and the spentcatalyst is regenerated. Catalyst undergoing regeneration in regenerator58 forms a dense phase bed having a level 62. Flue gas resulting fromcoke burned from the surface of the catalyst passes upwardly through theregenerator 58 and enters cyclone 63 wherein entrained catalyst isseparated from the flue gas and is returned to the regenerator densephase bed through dip leg 65. Cyclone 63, although represented as asingle vessel may, of course.

comprise an assembly of cyclones arranged in parallel and in series toeffect substantially complete separation of entrained solids from theflue gas. Effluent flue gas from cyclone 63 passes through line 64 intoplenum chamber 66. From the plenum chamber 66 the flue gas exits theregenerator vessel 58 through flue gas line 67 to vent facilities, notshown.

Regenerated catalyst is withdrawn from the bottom of the regenerator 58through line 71 and 72 at rates controlled by slide valves 73 and 74 tosupply hot regenerated catalyst to standpipes 22 and 1 1 as describedabove.

FIG. 2 of the drawings illustrates in detail a catalyst valve which maybe employed in the practice of the present invention and it is notintended to restrict the invention thereby since modifications may bemade within the scope of the claims without departing from the spiritthereof.

In FIG. 2 the catalyst valve is shown in an isometric view with oneclosure element for said valve shown in exploded view for clarity andthe other closure element shown in a normal relationship to theremaining elements of the catalyst valve.

Referring now to FIG. 2-of the drawing, a valve body 100 comprisingvertically disposed pipe having an open upper end and a lower end isattached to parallel support members 101 and 102. The upper end of thevalve body extends above support members 101 and 102 and the lower endof the valve body extends below said support members 101 and 102. Thelower end of valve body 100 is cut such that the lower end of said valvebody 100 defines two semi-elliptical spaces in allochiral relationship.Flapper vanes 103A and 103B comprising elliptical sections are locatedsuch that when said vanes 103A and 1038 are in a closed position thesemielliptical spaces defined by valve body 100 are completely covered.For clarity of detail flapper vane 103A is shown in an exploded view andflapper vane 1035 is shown in a closed position. Support members 101 and102 in a parallel relation comprise vertically disposed plates separatedby the width of the valve body 100. Support plate 101' has two holes inhorizontal alignment symmetrically disposed upon either side of thevalve body 100 and support plate 102 has two similar holes in axialalignment with the holes in support plate 101. Bearing rod 104Aextending through axially aligned holes is pivotally mounted betweensupport plates 101 and 102 upon one side of valve body 100 and bearingrod 104B extending through axially aligned holes is pivotally supportedbetween support members 101 and 102 upon the other side of valve body100. Connecting member 105A, attached to bearing rod 104A at a 90 angleextends downwardly and flapper vane 103A is attached thereto. Connectingmember 106A is attached to bearing rod 104A at a 90 angle and isangularly disposed to connecting member 105A. A weight 107A is attachedto connecting member 106A at a distance from the bearing rod 104A. Theweight 107A is disposed in relation to flapper vane 103A in such amanner that a rotational torque force is imposed upon the flapper vane103A. The rotational torque forces the flapper vane 103A against thevalve body 100 thereby covering the semi-elliptical section formed bythe lower part of the valve body 100.

As hereinbefore stated, flapper vane 1033 is an allochiral analogue offlapper vane 103A. By the same token, connecting member 105B, bearingrod 1048, connecting member 1068, and weight 1078 are the allochiralanalogues of connecting member 105A, bearing rod 104A, connecting member106A, and weight 107A respectively. It is to be understood that flappervane 103B operates in a manner analogous to the operation of flapper103A.

In a fluidized catalytic cracking process wherein a valve such as shownin FIG. 2 and described above is installed for passing catalyst from thereactor vessel 15 into the stripping zone 42, the inventory of catalystin the reactor vessel 15 exerts a pressure inside the valve body againstthe flapper vanes 103A and 1038. The torque force imposed upon theflapper vanes by weights 107A and 107B opposes the pressure imposed bythe catalyst. At equilibrium, an increase in catalyst inventory in thereactor vessel 15 will create a pressure inside the valve body 100sufficient to overcome the torque force imposed by the weights. Thus,the flapper vanes will be forced away from the valve body 100 andcatalyst will pass from the reactor vessel 15 through the valve body 100into the stripping zone 42. Sufficient catalyst will pass through thevalve body 100 to reduce the catalyst inventory of the reactor vessel 15until torque force imposed by the weight 107A will close the flappervanes 103A and 1038.

Under actual operating conditions in a fluidized catalytic crackingprocess wherein catalyst is continually discharging into the reactorvessel 15, the pressure exerted by the catalyst inventory will keep theflapper vanes continuously pushed away from the valve body 100 therebyallowing a continuous flow of catalyst from the reactor vessel 15 to thestripping zone 42. The torque force provided by the weights acts inopposition to the pressure exerted by the catalyst inventory such thatthe flapper vanes are maintained in a position such that the flow ofcatalyst through the valve body 100 is limited to the amount of catalystentering the reaction vessel 15. By selecting the length of theconnecting members 106A and 1068 and the weights of 107A and 1078, atorque force of known value may be imposed upon the flapper vanes 103Aand 1038. This known torque will then support a certain pressure exertedby the catalyst inventory upon the flapper vanes. Thus, by properlyselecting the length of the connecting members 106A and 1068, and theweights 107A and 107B a torque force may be provided which is sufficientto support the desired catalyst inventory in the reaction vessel.

We claim:

1. A fluidized catalytic cracking apparatus comprising:

a. a reaction vessel;

b. a stripping zone vertically mounted below said reaction vessel;

c. a counter weighted check valve connecting the bottom of the reactionvessel and the top of the stripping zone comprising a valve body havinga vertical opening therethrough, a flapper vane for closing the bottomof said valve body and a counter weight connected to said flapper vanefor maintaining said flapper vane in a closed position and supporting aselected inventory of catalyst in the reaction vessel;

d. a catalyst regeneration zone;

e. means for passing stripped catalyst from the stripping zone to theregeneration zone;

f. means for combining regenerated catalyst and hydrocarbon oil; and

g. means for passing such catalyst-hydrocarbon mixture into the reactionvessel.

2. A fluidized catalytic cracking apparatus comprising:

a. a reaction vessel; b. a stripping zone vertically mounted below saidreaction vessel; c. a counterweighted check valve connecting the bottomof the reaction vessel and the top of the stripping zone, said checkvalve comprising a valve body having an open top and a bottom shaped todefine an elliptical section,

support members horizontally disposed to said valve body and attachedthereto,

c. a flapper vane pivotally attached to said support wherein the valvebody is shaped to define two semielliptical sections in allochiralrelation; and wherein a weighted, pivotally mounted flapper vane coverseach semi-elliptical section.

",-- ;g" S'IATES PATENT om 'nm V t CER'IIFICA'IE OF CORRECTION PatentNo. 3,7 ,,-3 V Dated January 8, Inventofls) Dorrance P. Bunn Jr. and H.Blandin Jo es It is certified that error appears in the above-identifiedintent and that said Letters Patent are hereby'corrected as shown below:

"Henry Jones" should read--H. Blandin Jones--.

Signed and eealed this 29th day of October 1974 (SEAL) Attest:

MCCOY M. ,GIBSON JR, c. MARSHALL DANN Attesting Officer t v Commissionerofr P'atent s

2. A fluidized catalytic cracking apparatus comprising: a. a reactionvessel; b. a stripping zone vertically mounted below said reactionvessel; c. a counterweighted check valve connecting the bottom of thereaction vessel and the top of the stripping zone, said check valvecomprising a valve body having an open top and a bottom shaped to definean elliptical section, support members horizontally disposed to saidvalve body and attached thereto, c. a flapper vane pivotally attached tosaid support member and disposed to cover the elliptical section definedby the valve body, and a weight attached to the flapper vane for forcingthe flapper vane against the elliptical section; d. a catalystregeneration zone; e. means for passing stripped catalyst from thestripping zone to the regeneration zone; f. means for combiningregenerated catalyst and hydrocarbon oil; and g. means for passing suchcatalyst-hydrocarbon mixture into the reaction vessel.
 3. Thecounterweighted check valve of claim 2 wherein the valve body is shapedto define two semi-elliptical sections in allochiral relation; andwherein a weighted, pivotally mounted flapper vane covers eachsemi-elliptical section.